Does emergency contraception work all the time?
No. According to the U.S. Department of Health & Human Services, emergency contraceptive pills (ECPs) that contain both estrogen and progestin are about 75 percent effective at keeping a woman from getting pregnant. In other words, if 100 women had unprotected sex (sex without using birth control) in the fertile part of their cycle (when an egg is most likely to leave the ovary), about 8 of those women would become pregnant.
If all 100 women took combined emergency contraceptive pills, only 2 would become pregnant. ECPs containing only progestin are about 89 percent effective. If those same 100 women took progestin-only ECPs, only 1 would become pregnant. The IUD is 99.9 percent effective. If 1,000 women had an IUD put in, only one would become pregnant.
According to the U.S. Department of Health & Human Services, the sooner your wife takes emergency contraception after sex, the better her chances it will work.